Aircraft windows are known in the state of the art that are constituted by an inner pane and an outer pane, both mounted by means of a sealing ring on a window frame intended to be mounted flush on the fuselage of the aircraft.
Among the functions of the window there can be mentioned:                allowing passengers to look out, in particular during takeoff;        maintaining the pressure inside the cabin during the flight, even at high altitude.        
In the state of the art, the outer pane is subjected to a significant pressure difference in flight between the relatively low outer pressure and the inner pressure, which is substantially that of the aircraft cabin.
In order to avoid stressing the inner pane, the latter is pierced so as to equalize the pressure between the cabin itself and the gap or volume constituted between the inner and outer panes of the window.
In a known manner, the outer pane is designed so as to remain flush with the outer surface of the fuselage when the aircraft is situated at ground level and, as a result, produce a minimal adverse effect from an aerodynamic point of view.
Owing to the pressure difference between the ground and in particular the cruising altitude of the aircraft, the outer pane tends to deform toward the outside of the fuselage. In order to ensure that the additional drag effect does not exceed a certain limit, care must be taken that the maximum extra thickness introduced by the outer pane with respect to the natural surface of the fuselage also does not exceed a certain limit. Under given flight conditions, the thickness of the pane must therefore be increased, in particular by using a greater quantity of material so as to ensure that its deformation toward the outside of the fuselage does not exceed an acceptable tolerance, in particular vis-à-vis the aerodynamic drag effect.
Consequently, the outer pane must be thicker than the inner pane. In an embodiment of the state of the art, the thickness of the outer pane is 11 mm and that of the inner pane is 5.25 mm. In this embodiment, the weight of all of the outer panes of the aircraft together is 200 kg.
Moreover, the operating cost of an aircraft is a direct function of the mass of the aircraft, since the greater the mass of the aircraft, the more fuel is needed. Consequently, reducing the operating cost in flight requires a reduction in the mass of the aircraft. It will be noted that the drag effect also increases the resistance in flight and therefore the quantity of fuel required.
The abovementioned extra thickness of the outer pane of the aircraft windows specifically increases the mass of the aeroplane.